1. Field of the Invention
The present invention relates to a delayed-tack adhesive article having an adhesive layer which is non-tacky at ordinary temperatures but can develop tack by heating and is kept in a tacky state for a long time after the completion of the heating; and a delayed-tack adhesive composition useful for the production of the adhesive article.
2. Description of Related Art
A delayed-tack adhesive composition essentially comprises a thermoplastic polymer (or resin), a plasticizer being solid at ordinary temperatures, and optionally a tackifier. Among these components, the thermoplastic polymer serves as the basic component for developing tackiness and adhesiveness. On the other hand, the plasticizer is molten by heating to swell or soften the polymer, though it is solid at ordinary temperatures and hence cannot impart plasticity as it is to the polymer. In an adhesive article produced by the use of a composition comprising a thermoplastic polymer and a plasticizer being solid at ordinary temperatures, which exhibit the above properties respectively, the adhesive layer of the article is non-tacky at ordinary temperatures, but can develop tack by heating. Further, the tackifier serves to further improve the tack performance.
Up to this time, ethylene-vinyl acetate copolymers, styrene-butadiene copolymers and so on have occupied the mainstream of the thermoplastic polymer used in delayed-tack adhesive articles and delayed-tack adhesive compositions. These copolymers exhibit poor tack strength for adherends made of non-polar materials such as polyethylene, polypropylene and so on, though they exhibit relatively high tack for adherends made of polar materials such as glass, stainless steel and so on. Therefore, there was a problem that when an adhesive article having an adhesive layer comprising an ethylene-vinyl acetate copolymer or a styrene-butadiene copolymer was applied to an adherend made of such the non-polar material, the adhesive article peeled off the adherend within a short time after the application. Further, with respect to such an adhesive article, it has also been known that when the adhesive composition for forming the adhesive layer of the article takes some formulation, the resulting adhesive article is liable to cause blocking, though it is excellent in adhesiveness [see Japanese Patent Publication-A No. 63-172784 (published on Jul. 16, 1988)].
Under these circumstances, there have been proposed a delayed-tack adhesive excellent in tack characteristics and blocking resistance which comprises a thermoplastic resin (except ethylene-vinyl acetate copolymers) having a glass transition temperature of -5.degree. C. or above, a crystalline plasticizer and a tackifier [see Japanese Patent Publication-A No. 56-57233 (published on Mar. 1, 1994)]; and another delayed-tack adhesive comprising a thermoplastic resin, a tackifier, a crystalline plasticizer and a colloid [see Japanese Patent Publication-A No. 6-100848 (published on Apr. 12, 1994)]. In the Examples of these patent documents, however, only stainless steel is used as the material of the adherend in the evaluation tests on adhesive strength and holding power, and therefore the above adhesives are still unknown in the adhesive strength and holding power for adherends made of non-polar materials. With respect to the thermoplastic resin as the adhesive component, on the other hand, the former document only stipulates that it must have a glass transition temperature of -5.degree. C. or above and that it must be one other than ethylene-vinyl acetate copolymers. That is, both documents are silent on the detailed relationship between the kind of the thermoplastic resin and the kind of the material of the adherend.
As described above, there has not been found as yet any delayed-tack adhesive article which can be suitably applied even to an adherend made of a non-polar material and exhibits performances (such as tack, adhesiveness and blocking resistance) satisfactory enough for practical use. Actually, the use application of most of the labels which are each produced by coating a substrate with a delayed-tack adhesive composition and are put to practical use is limited to glass cases, and scarcely any such label applicable to plastic cases has been put to practical use. In other words, the delayed-tack adhesive article of the prior art produced by the use of the above thermoplastic resin had a problem that the use application thereof was limited because of the unpractically poor performance thereof for adherends made of plastics.